... you must not give back the attacking weapon for another attack.
And you must not hold the weapon which is carrying the spirit of the attacker longer then needed ...
I think the weapon of an attacker / enemy doesn't deserve any respect due to this understanding. And one just has to get get rid of it.
And I added:
I don't know how the sword of the enemy was treated in "old japan"?
I only know reishiki concerning ones own sword or the sword of a friend.
I don't know any traditional way how to treat the sword of an enemy.
I once heard that you would break the blade?